1. Field of the Invention
The present invention relates to a polymerization process for polyesters and more particularly to a process for producing a polyester, which comprises thermal polymerization of a bis(.omega.-hydroxyalkyl)terephthalate or an oligomer thereof in the form of fine particles emulsified in a silicone oil.
2. Description of the Prior Art
The bulk melt polymerization process has been most commonly applied to produce polyesters which are typified by polyethylene terephthalate. However, this process has several drawbacks such that as the molecular weight of polyester increases under polymerization, the melt viscosity thereof increases and more agitation power becomes necessary, that the increased melt viscosity makes worse the efficiency of heat transfer through walls of the polymerization vessel and hence the polymer is liable to undergo thermal degradation locally, and that a long time is required for taking out the polymer and this brings about difference in the degree of polymerization between the initially discharge polymer and the finally discharged polymer.
To overcome these drawbacks, the present inventors studied polymerizations of bis(.omega.-hydroxyalkyl)terephthalates (hereinafter referred to as BHT) or oligomer thereof. As a result, a very effective polymarization process for the polyester has been found out, which comprises dispersing BHT or an oligomer thereof with a dispersion stabilizer to a finely emulsified state in a silicone oil which is a medium inert to BHT or to the oligomer, and reacting the BHT or the oligomer to remove the alkyleneglycol therefrom. Thus the present invention has been accomplished.